1. Field of the Technology:
The present invention relates to electronic flash equipment for photography, and particularly to an improvement in a part for time control of light radiation of an electronic flash tube.
2. Description of the Prior Art:
When there is not enough light to take a photograph, e.g., in a dark interior or in the evening or night, electronic flash equipment is widely used to provide enough light to an object. For this purpose, electronic flash equipment which has various functions is developed and is practically used.
One of these functions is to detect a reflected light from the object, and the light which is applied to the object is automatically controlled to an adequate value based on this detected light. In such an automatic light controlled electronic flash equipment, as is well known, there are two methods to control light. One method, named a series type, stops discharge of a main capacitor during flashing. The other method, named a parallel type, bypasses the electric energy charged in the main capacitor to another element instead of the flash tube during flashing.
A circuit of a conventional series type automatic light controlled electronic flash equipment is shown in FIG. 1, wherein a flash light tube 2 is connected to a SCR 3 in series. The electric charge of the main capacitor 1 is supplied to the flash light tube 2 which is excited by a trigger circuit 4, and the flash light tube 2 radiates the light toward the object.
The reflected light out of the object is detected by a light detector 8. When the detected light reaches a predetermined value, the light detector 8 triggers a SCR 9 and turns it on. Since a capacitor 5 is connected to ground through the SCR 9, this capacitor 5 is charged and an electric potential of an anode of the SCR 3 is pulled down. Upon application of inverse voltage across the anode and the cathode of the SCR 3, the SCR 3 turns off, and the light radiation is stopped.
In this conventional electronic flash, as is shown in FIG. 2, after the sum of the reflected light from the object reached the predetermined value at the time T, it is desireable for the light radiation to stop immediately. However, the light radiation of the flash light tube does not vanish completely, because the current from the main capacitor 1 continues to flow through the flash light tube 2, the capacitor 5 and the SCR 9, for a period after the time T. As a result, the flash light tube 2 radiates unnecessary light during an above-mentioned period. Thus, the sum of the applied light to the object exceeds the adequate value for photography. This surplus part of the light is shown by hatching in FIG. 2.
Furthermore, in case that the flash light equipment is used to repeatedly radiate the flash light within short intervals, a time constant which produced by the capacitor 5, resistor 6 and 7 must be short enough in order to completely turn off the SCR 3 at every flash radiation. If the resistor 6 is of a small value, the SCR 9 will not turn off.
In case the focusing in a camera is very difficult due to darkness of scenery, there is a method to illuminate the object by repeated flashing of a flash light. The electronic flash equipment in accordance with this method has a small auxiliary subcapacitor for charging with the electric energy, whereby the repeated flash lights are radiated by the cyclic triggering of the flash tube. An amount of the radiated light depends on the capacitance of the capacitor and the charging voltage. In the abovementioned prior art, the relation between the capacitance of the subcapacitor, charging voltage and the period of the trigger circuit must be sufficiently accurately adjusted in respective suitable values. However such adjustment is very difficult because the amount of the light is minute the capacitance of the capacitor is too small in order to decrease the charging time. In case that the capacitance is too large, the charging time become longer, and hence cyclic time of the trigger circuit must be longer. Furthermore, recently a small dry battery, such a UM-3 type, is usually used as the power source. These small dry-cell batteries have a little electric energy, and when the dry battery is consumed, the time which is required to charge the capacitor becomes longer. On the other hand, if the cyclic time of the trigger circuit does not change by the exhaustion of the dry battery, the flash light equipment will be operated by a lower charging voltage, and the amount of the light necessary to observe the object will not be obtained.